The present invention relates to a traction drive transmission and more particularly to a hydraulic control system performing a lock-up control and a shift control.
Japanese Utility Model Application First (unexamined) Publication No. 64-27563 discloses a hydraulic control system performing a lock-up control and a shift control of a traction drive transmission. For the lock-up control, a lock-up solenoid and a lock-up control valve are arranged, and for the shift control, a shift control valve and a stepper motor for the shift control valve are arranged. According to the hydraulic control system, the lock-up control is performed via the lock-up solenoid under the control of a control unit and the shift control is performed via the stepper motor under the control of the control unit. This known hydraulic control system involves a potential problem that if the lock-up solenoid should malfunction, a lock-up mechanism might be kept in its lock-up state, causing an engine stall at low vehicle speeds.
An object of the present invention is to improve the hydraulic control system such that the lock-up mechanism is held to a lock-up release state to prevent the above-mentioned potential problem.